a. Field of the Invention
The present invention relates to an electronically driven pressure boosting system that is used to boost the torque output of an internal combustion engine.
b. Related Art
One way to boost the torque and peak power provided by a reciprocating piston internal combustion engine, is to use a pressure boosting device to increase the mass airflow into the engine. The increased air supply then permits a greater amount of fuel to be combusted in each ignition event.
Examples of pressure boosting devices include turbochargers and superchargers. A turbocharger is driven entirely or partly by energy in the exhaust stream. This is an efficient use of otherwise mostly wasted energy, but such devices suffer from the limitation that the boost is not available or significant at low engine speeds (rpms). Often, a driver may demand high torque from an engine at low rpms, for example at the start of an overtaking manoeuvre. If the pressure boost device is driven only by exhaust gasses, then boosted torque will not be available at low rpms.
One way of dealing with the limitation is to provide an electrical motor connected to the turbocharger, which is energised when the turbo boost is insufficient. This type of electrically driven pressure boosting device is, however, expensive in terms of hardware cost. Another solution is to use a supercharger, that is, a compressor device that is driven by means other than an exhaust gas turbine, for example via a mechanical linkage to the engine, or by an electrical motor driven from the vehicle battery and/or battery charging system. Mechanical supercharger systems can however, be mechanically bulky and expensive. Electrically driven supercharger systems provide a lower cost and compact solution, but can require a significant amount of electrical energy when driven, for example, up to three times the current which can normally be supplied by a typical motor vehicle 12 volt battery. Motor vehicle alternators are typically specified to provide either all or most of the power requirement for the entire vehicle, the battery only being used to store sufficient electrical power to start the vehicle engine and occasionally deliver power when the accessory load exceeds the alternator output. Typical European vehicle alternators are specified to provide about 130 A of current, while an electrically powered supercharger can require in excess of 300 A. An alternator able to supply this much current is significantly more expensive, heavy and bulky than a conventional alternator.
Because the pressure boosting device cannot be 100% efficient, there will also be inevitable electrical and mechanical losses associated with the device, that can place significant mechanical and thermal stress on components within the device.
The expense of increasing the capacity of the vehicle battery and charging system, or the dealing with inherent thermal and mechanical limits of components within the pressure boosting device, to meet any level of driver demand can easily outweigh the benefits of using an electrically driven pressure charging device. Therefore it is important to drive such a device in an efficient manner, and within the limits of the vehicle electrical power supply, and thermal and mechanical limits of the device itself. At the same time, it is important to maximise the torque boost benefit perceived by the driver over as wide a range of driving conditions as possible. Because the level at which an electrically driven pressure boosting device is driven is essentially independent from the engine operating speed, it is therefore necessary to devise an appropriate control system for operating the pressure boosting device that takes account of the system""s limitations.
It is an object of the present invention to provide a convenient and economical electrical pressure boosting device and method for increasing the torque available from an internal combustion engine.
According to the invention, there is provided an air charge boosting system for an internal combustion engine, the system comprising an electrically driven pressure charging device, an electrical supply system for providing electrical power to drive the pressure charging device including a battery and an engine-driven battery recharger, and an electronic control system for controlling the operation of the pressure charging device and the electrical supply system, wherein the electronic control system is arranged to:
a) determine one or more allowable operating limits to the operation of the pressure charging device based on the state of the electrical supply system; and
b) drive the pressure charging device using the engine-driven battery recharger when the state of the electrical supply system is within an acceptable range; and
c) isolate at least partially the battery from the engine-driven battery recharger and drive the pressure charging device using the battery when the state of the electrical supply system is not within an acceptable range.
Also according to the invention, there is provided a method of operating an air charge boosting system for an internal combustion engine, the system comprising an electrically driven pressure charging device, an electrical supply system for providing electrical power to drive the pressure charging device including a battery and an engine-driven battery recharger, and an electronic control system for controlling the operation of the pressure charging device and the electrical supply system, wherein the method comprises the steps of using the electronic control system to:
i) calculate one or more allowable operating limits to the operation of the pressure charging device based on the state of the electrical supply system; and
ii) drive the pressure charging device using the engine-driven battery recharger when the state of the electrical supply system is within an acceptable range; and
iii) isolate at least partially the battery from the engine-driven battery recharger and drive the pressure charging device using the battery when the state of the electrical supply system is not within an acceptable range.
Because the pressure charging device is operated from the battery when the state of the electrical supply system is not within an acceptable range, other electrical components may be operated from the battery recharger. This helps to avoid problems due to operation of the device beyond the allowable limits such as a drop in voltage available to drive other electrical components, thereby allowing more economical system design. This will in general also result in reduced electrical power consumption for the device. Both of these benefits contribute towards reducing system mass, volume and cost, and improving the electrical efficiency of the device.
The state of the electrical supply system may be a battery state of charge. Preferably, the acceptable range for the battery state-of-charge is a predetermined minimum supply voltage for the battery.
Additionally or alternatively, the state of the electrical supply system may be the level of the electrical load on the battery recharger, particularly a maximum acceptable electrical load.
In a preferred embodiment of the invention, one or more additional non-electrical allowable operating limits to the operation of the pressure charging device are also determined. The operation of the pressure charging device is then limited according to the determined allowable operating limit that places the greatest restriction on the operation of the electrically driven pressure charging device. Therefore, the operation of the pressure charging device remains within all allowable operating limits.
The additional operating parameter may be a thermal parameter of the pressure charging device, in which case the determined allowable operating limit may be a maximum thermal limit.
It is particularly advantageous if the calculation of the allowable operating limit involves a calculation of both a soft operational limit and a hard operational limit. The operation of the electrically driven charge boosting device then being limited when the corresponding operating parameter exceeds the soft operational limit. The limitation is then such that the operating parameter does not at a later time exceed the hard operational limit. Such limitation may takes several forms, but so that operation of the pressure boosting device is not suddenly or unexpectedly limited, it is preferred if the driven boosting device is progressively limited as the corresponding operating parameter approaches the hard operational limit.
For example, on a hot summer day with a hot engine, the thermal limits may be reached well before the battery state-of-charge starts to limit the amount of current that may be provided by the battery, while on a cold winter night, the reverse may be the case.
The air boosting system may comprise an electronically controlled throttle for controlling engine aspiration. The system may then: set the throttle position to regulate the aspiration of the engine; determine the actual air charge according to engine operating conditions; compare the desired air charge with the actual air charge; and drive the pressure charging device to boost aspiration of the engine in accordance with the comparison between the desired air charge and the actual air charge only when the throttle is wide open and when the torque demand cannot be met by natural aspiration alone.
The engine torque output at any given engine speed is then controlled by the throttle at throttle positions below wide open throttle. Once the throttle position reaches wide open throttle, the control system operates the pressure charging device according to the comparison between the desired air charge and actual air charge, for example in such a way as to reduce a difference between these air charges to zero in the case of steady state operation of the pressure charging device.
The pressure charging device operation is therefore limited to regions where engine torque demand cannot be met with natural aspiration alone. Furthermore, the efficiency of assisted aspiration is increased by avoiding restriction by a partly open the throttle, and by matching the actual air charge to the desired air charge by the electrical control of the pressure charging device.
The desired air charge may be calculated according to current engine operating conditions, including a current air charge. The current air charge may be measured directly, for example by means of an air flow sensor, or indirectly, for example by engine inlet air pressure sensor and an engine speed sensor.
The overall efficiency of assisted aspiration is also increased by matching the actual air charge to the desired air charge by the electrical control of the pressure charging device.
The electronic control system preferably provides closed-loop control over the operation of the electrically driven pressure charging device by monitoring the current air charge. Other engine operating parameters may also be measured, for example engine speed, engine temperature. It may also be useful to measure ambient conditions such as ambient temperature and relative humidity. Any such parameters which may have an effect on the calculation of the desired air charge may be included in this calculation.
The engine torque demand may be calculated according to a driver demand, for example the output from an electronic accelerator pedal sensor. The engine torque demand may be modified according to other inputs, for example an input from a traction control system, a clutch pedal sensor, a collision warning system, or a system for controlling the operation of an automatic gearbox.
Preferably, the desired air charge is a measure of desired air pressure and/or desired mass air flow.
The pressure charging device may be any type of device for boosting the air mass flow into the engine, whether partially or wholly driven by an electric power supply, but is most preferably an electrically driven supercharger with a number of rotary impeller vanes. The geometry of the each vane is fixed, and the actual air charge delivered through the supercharger is determined by the rotation speed of the impeller vanes.